SMLoc itself encapsulates just a pointer, so there is no need to pass or
return it by reference.

This PR passes the VFS down to `llvm::cl` functions so that they don't
assume the real file system.

Since `raw_string_ostream` doesn't own the string buffer, it is
desirable (in terms of memory safety) for users to directly reference
the string buffer rather than use `raw_string_ostream::str()`.

Work towards TODO comment to remove `raw_string_ostream::str()`.

For some reason this was missing from STLExtras.

FileCheck currently compiles a regular expression of the form
`Prefix1|Prefix2|...` and uses it to find the next prefix in the input.

If we had a fast regex implementation, this would be a useful thing to
do, as the regex implementation would be able to match multiple prefixes
more efficiently than a naive approach. However, with our actual regex
implementation, finding the prefixes basically becomes O(InputLen *
RegexLen * LargeConstantFactor), which is a lot worse than a simple
string search.

Replace the regex with StringRef::find(), and keeping track of the next
position of each prefix. There are various ways this could be improved
on, but it's already significantly faster that the previous approach.

For me, this improves check-llvm time from 138.5s to 132.5s, so by
around 4-5%.

For vector-interleaved-load-i16-stride-7.ll in particular, test time
drops from 5s to 2.5s.

Some cl::ZeroOrMore were added to avoid the `may only occur zero or one times!`
error. More were added due to cargo cult. Since the error has been removed,
cl::ZeroOrMore is unneeded.

Also remove cl::init(false) while touching the lines.

Also fix directives not covered by D125604.

Discourse thread: https://discourse.llvm.org/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D121332

Remove superfluous variable initialization, the variable is assigned by both paths immediately afterward.